This invention relates to a method of purifying sugar juice which has been prepared by extraction of a sugar beet material wherein undissolved components are mechanically separated from the sugar juice and high molecular components are subsequently separated from the juice thus obtained.
In the production of sugar from sugar beets the beets are sliced and the slices thus obtained are extracted with warm water, e.g., having a temperature of 70.degree. C., so as to form a sugar juice and exhaust slices (pulp). The sugar juice, which is then separated from the pulp, contains in addition to sucrose various non-sugars, e.g. organic and inorganic salts, amino acids, dyes and high molecular substances, such as protein and pectin.
The sugar juice thus obtained is then purified to remove the non-sugars.
In a conventional juice purification method lime (CaO) and carbon dioxide (CO.sub.2) are added, the carbon dioxide being prepared by heating limestone in a lime kiln based on the use of solid fuel, e.g., cinders.
The addition of lime and carbon dioxide results in the formation of a precipitate (sludge) consisting of calcium carbonate and part of the above-mentioned non-sugars. The sludge is removed by filtration, e.g., on a rotating vacuum, filter. The dewatered sludge thus obtained is optionally used as a soil-improving material.
The filtrate obtained by removing the sludge by filtration still contains some lime (CaO) and, therefore, additional amounts of carbon dioxide and, optionally, soda may be added, and the pH may also be adjusted to a value of between 9.0 and 9.2 to form additional sludge, which is subsequently removed by filtration.
The filtrate thus obtained may be treated with sulfur dioxide (SO.sub.2) before it is worked up in a conventional manner, e.g., to form crystalline sugar.
The specification of British Pat. No. 1,361,674 discloses a method in which the conventional juice purification method has been replaced by a purification procedure in which the initial mechanical removal of undissolved material, e.g., by normal filtration, is followed by a step in which the juice is subjected to ultrafiltration. The ultrafiltration is effected with a membrane of the type which permits the passage of water and sucrose molecules but rejects higher molecular compounds. After an initial ultrafiltration, water may be added to the concentrate, which is then subjected to a further ultrafiltration. This procedure is optionally repeated.
Finally, the permeates thus obtained are subjected to one or more after-treatments in the form of a chemical treatment, a conventional filtration, an ion exchange treatment or a hyperfiltration. In connection with these treatments the pH value of the sugar juice may be adjusted to 6-11.5, e.g., by the addition of lime (CaO).
The above-mentioned prior art method does not constitute an economically satisfactory manner of providing a sugar juice which is sufficiently colourless to allow high quality sugar, i.e. white sugar, to be prepared therefrom. It is assumed that this is due to some low molecular dyes which together with the sucrose molecules pass through the membrane and into the permeate during the ultrafiltration step.
The object of the present invention is to provide a method for the purification of sugar juice which does not suffer from the above-mentioned drawback and which is simpler and less expensive than the conventional lime purification method.